1. Field of the Invention
The present invention relates generally to an exhaust gas recirculation (EGR) system for feeding engine exhaust gas into an engine intake system to reduce the production of nitrogen oxides (NOx) and particularly to an EGR system of this type which is provided with valve means for admitting both the atmospheric air and an engine suction vacuum into the vacuum chamber of the diaphragm unit for the EGR control valve during engine warming-up operation to have the EGR control valve control the flow of recirculated engine exhaust gas to a proper value less than during engine normal operation after the completion of engine warming-up.
2. Description of the Prior Art
As is well known in the art, an exhaust gas recirculation (EGR) system for an internal combustion engine serves to reduce the production of nitrogen oxides by controlling the maximum combustion temperature in an engine combustion chamber a certain degree by recirculating or feeding into an air-fuel mixture drawn by the engine a portion of exhaust gas emitted from the engine. Although the effect of reducing the production of nitrogen oxides is heightened by increasing the flow of engine exhaust gas recirculated into the air-fuel mixture, the fuel economy and operating performance of the engine are degraded. For solving such a problem, the EGR system is usually provided in the EGR passageway with an EGR control valve operated in response to variations in a suction vacuum of the engine and controlling the flow of recirculated engine exhaust gas in accordance with an operating condition of the engine. However, when the engine exhaust gas is fed into the engine air-fuel mixture under the control of the EGR control valve during engine warming-up condition in which the engine is not sufficiently warmed up, the combustion of the air-fuel mixture is degraded not only to produce engine exhaust gas containing large quantities of hydrocarbons (HC) and carbon monoxide (CO) but also to considerably reduce the output performance and fuel economy of the engine. As a solution of such a problem, when a restriction is provided in a passage conducting a suction vacuum into a vacuum chamber of the EGR control valve, the restriction merely serves to conduct the suction vacuum into the vacuum chamber with a time delay during the warming-up operation of the engine and can not be expected to control the flow of recirculated engine exhaust gas to a value proper for the engine warming-up operation. On the contrary, since the suction vacuum is conducted into the vacuum chamber with a time delay even after completion of the warming-up of the engine, the response of the EGR control valve to variations in the suction vacuum is delayed during normal operating conditions of the engine to lower the effect of reducing the production of nitrogen oxides. Thus, a conventional EGR system has been arranged to stop the EGR during the engine warming-up operation because of difficulty in controlling the flow of recirculated engine exhaust gas. Accordingly, the conventional EGR system has had a drawback that it can not display the effect of reducing the production of nitrogen oxides during the engine warming-up operation.